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Lignin complexity: fundamental and applied issues

Göran Gellerstedt

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Content

• The lignin structure in wood

• Lignin chemistry in pulping

• Technical lignins

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Content

• The lignin structure in wood

• Lignin chemistry in pulping

• Technical lignins

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Spruce: C9H8.62O2.48(OCH3)0.94 Phenolic OH: 20-30%

Birch: C9H8.59O2.86(OCH3)1.52 Phenolic OH:

Milled Wood Lignin

Percent of total linkages Linkage

type

Dimer structure

Softwood Hardwood

-O-4'

-O-4'

-5'

5-5'

4-O-5'

-1'

'

Arylglycerol--aryl ether

Noncyclic benzyl aryl ether

Phenylcoumaran

Biphenyl

Diaryl ether

1,2-Diaryl propane

Pinoresinol/lignan type

50

2-8

9-12

10-11

4

7

2

60

7

6

5

7

7

3

Ref., Adler, 1977

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Monomer yield on thioacidolysis(theoretical: ~4700-5500 mol/g)

Sample Yield of the main monomer(s),

mol/g Klason lignin

Content of phenolic OH,

Number per 100 C9-units

Spruce wood

Spruce wood (preswollen)

Spruce MWL

Spruce TMP (preswollen)

Birch wood (preswollen)

Birch MWL

Aspen wood (preswollen)

Aspen MWL

1332

1682 (31%)

986

1498

672 (G) + 2318 (S) = 2990 (63%)

403 (G) + 809 (S) = 1212

866 (G) + 1942 (S) = 2808 (58%)

609 (G) + 863 (S) = 1472

n.a.

10-13

20

14

7.6

n.a.

10

n.a.

O OCH3Lignin

O

HO

R

HO

H3CO

Lignin

R

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Mechano-chemical cleavage of -O-4 structures in milling

OOCH3

CHOH

L

CHCH2OH

O L

H3CO

O

OCH3

CHOHCHCH2OH

L

O

OCH3

+

L

M. E.

+H-H

OOCH3

C OCH2

CH2OH

LOH

OCH3

L

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SEC of thioacidolysis products from spruce, eucalyptus and birch wood

0

0.2

0.4

0.6

0.8

1

1.2

20 25 30 35 40

MonomersDimers

Trimers

OligomersSpruce

Eucalyptus/Birch

Time, min

Absorbance

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•Endoglucanase (Novozyme 476)

•Action of urea - Breaks down the crystallinity of the cellulose by forming hydrogen bonds between the microfibrils

- Dissolves any material containing > ~50% lignin

- Removes enzyme contamination from the fibres

•Action of alkaline borate solution- Dissolves all remaining components

Dissolution of wood/pulp fibres by the use of enzyme

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Types of LCC isolated from spruce wood meal

Type of Lignin-Carbohydrate Complex, LCC Lignin yield, %

GalactoGlucoMannan - Lignin

Glucan - Lignin

GlucoMannan - Lignin

Xylan - Lignin

8

4

48

40

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Dimer Monomer

SEC of acetylated thioacidolysis products from spruce LCCs

Xylan-rich LCC (40% lignin on wood)

Glucomannan-rich LCC(48% lignin on wood)

Wood

Response

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Suggested lignin structures in spruce wood

OH

OH

O OMe

HO

OMeO

OH

O

HO

OMe

O

OH

MeO

MeO

O

HO

O

OH

OMe

MeO

OLignin

Xylan

Linear xylan-lignin

Branched glucomannan-lignin

O

Lignin

O

O

O

O

O

OO

O

O

HO OH

OO

O

O

O

O

O

O

CHO

O

Lignin

CH2OH

O

O

O

O

O

Lignin

HO

OCH3

H3CO

OCH3

H3CO

H3CO

OCH3

OCH3

OCH3

H3CO

H3CO

H3CO

OCH3

OCH3

OCH3

H3CO

OCH3

OCH3

OCH3

H3CO

OCH3

H3CO

OCH3

OCH3H3CO

OCH3

HO

HO

HO

OH

OH

OH

OH

HO

HO

Glucomannan

HO

HO

OHHO

OH

OHOHOH

OH

OH

OHHO

HO

HO

HO

OH

OH

OH

HO

OHOH

OH

OH

OH

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S/G ratios in hardwoods

Wood species S/G-ratio Method Reference

Birch

Birch

E. globulus

E.globulus

E. grandis

3.8

3.7

5.3

4.8

3.6

Thioacidolysis

Nitrobenzene

Thioacidolysis

Pyrolysis

Pyrolysis

Gellerstedt et al, 2007

Chen, 1992

Gellerstedt et al, 2007

Gutierrez et al, 2007

Gutierrez et al, 2007

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G-units/S-units in white birch wood

Morphological Differentiation Guaiacyl/Syringyl

Fibre, S2-layer

Vessel, S2-layer

Ray parenchyma, S-layer

Middle lamella (fibre-fibre)

Middle lamella (fibre-vessel)

Middle lamella (fibre-ray)

Middle lamella (ray-ray)

12 : 88

88 : 12

49 : 51

91 : 9

80 : 20

100 : 0

88 : 12

Ref. Saka and Goring, 1988

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The lignin structure in hardwoods …

contains a high proportion of S-units

which results in a high percentage of

linear lignin – unevenly distributed

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MS-identification of lignin fragment from E. globulus lignin

HOOCH3

H3CO

OHO

H3CO

H3COOH

O

OCH3

H3CO

OH OH

O

O

O

OH

O

OCH3

OCH3

HO

OH

OH

OCH3

HO

OCH3

H3CO

Evtuguin et al, 2003

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Lignin in annual plants

Origin Lignin content H:G:S

Flax

Sisal

Wheat straw

Rice straw

2.9 (+ 1.6)

10.8 (+ 3.0)

16.0

6.1

57:33:11 (pyrolysis)

1:20:79 (pyrolysis)

5:49:46 (thioacidolysis)

15:45:40 (thioacidolysis)

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Content

• The lignin structure in wood

• Lignin chemistry in pulping

• Technical lignins

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Dissolution of lignin and carbohydrates in kraft pulping

Residual lignin; removed by bleaching

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Degree of delignification for different wood species

Pulp type Kappa No Lignin

kappa

Delign.

degree

Pine

Birch

E. globulus

28.0

16.5

15.9

24.6

4.0

5.7

94.0

98.2

97.5

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Kraft pulping of birch and E. globulus respectively to similar kappa numbers

E. globulus

Birch

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-O-4 structures in wood and pulp based on thioacidolysis

0

500

1000

1500

2000

2500

3000

Birch B pulp Euc E pulp

G

S

(birch and eucalyptus)

Degradation product, mol/g of lignin

Klason lignin, %: 16.6 0.6 18.3 0.9

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Size exclusion chromatography (SEC) of lignin degradation products

(no ”residual lignin” present in wood)

Methodology

•Thioacidolysis of wood/pulp•Acetylation•SEC in tetrahydrofuran

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Suggested mode of formation of radical coupling products in kraft pulping

O

H3CO

Lignin

O

OCH3

R

SS

SS

SS

SS

SS

S SSS

SS

O

H3CO

Lignin

O

OCH3

R

SS

SS

SS

SS

O

H3CO

Lignin

O

OCH3

R

O

H3CO

Lignin

HO

OCH3

R

OOCH3

OH3CO

H

Lignin R

H H

Low reactivity due to H-bonding

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Principles in the steam explosion process(Conditions: ~190-240 oC, 1-5 min)

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Chemical composition before and after steam explosion

Substantial removal of hemicelluloses and extractives: SO2SE > TwoSE > OneSE

0

20

40

60

80

100

Wood SO2SE OneSE TwoSE Wood SO2SE OneSE

Lignin

Extractives

(Ara)-xyl

(Gal)-Glu-man

Glucan Spruce samples Birch samples

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Lignin isolation yield (hardwoods)

SO2SE > OneSE

0

20

40

60

80

100

120

SO2SE OneSE SO2SE OneSE

Residual

Extractable,NaOH

(missing lignin from aspen highly soluble lignin)

Birch samplesAspen samples

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SEC of acetylated lignin from steam exploded aspen wood

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Degradability by thioacidolysis/SEC analysis

Condensation less degradability

Spruce

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Degradability by thioacidolysis/SEC analysis, SE aspen

SESO2SE

monomers

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Steam explosion chemistry

O

OCH3

OHO

Lignin

HO

OCH3

LigninO

OCH3

HO

HO

Lignin

O

H3CO

Lignin

O

OCH3

O

Lignin

HO

OCH3

Lignin

O

OCH3

O

Lignin

H3CO

Lignin

Lignin

OH3CO

OH

O

OCH3

OH

Lignin

O

OCH3

OH

Lignin

O

High temperature

Hydrolysis, H+Condensation

Acidolysis

Stabilisation

O

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Content

• The lignin structure in wood

• Lignin chemistry in pulping

• Technical lignins

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Biomass tree showing the main chemical outlets

Ref. Rintekno oy, 1984

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Highest-value lignin uses to show greatest future rise (W. Glasser)

As structure of lignin yields to advances in analytical techniques, new markets are projected in adhesives, foams, films, coatings and plasticsRef: C&EN 1984

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The Biorefinery Concept

• Production of large volumes of ethanol will be necessary in a short term

• New separation process(es) for lignocellulosics required

• New chemistry based on carbohydrates will be developed

• Lignin for fuel – and for chemicals• On a longer term, gasification of biomass to

syngas (biodiesel) will be developed

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Indicative targets for the share of biofuel in the EU

• 2005: 2% (not achieved)

• 2010: 5.75% (will probably not be achieved)

-------------------

• 2007: New energy policy document setting a minimum requirement at 10% by 2020

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From biomass to liquid fuels

• Biodiesel from oils and fat; rapeseed etc – esterification with methanol

• Biochemical pathways to ethanol; 1) Sugar beet etc – sugar-fermentation 2) Starch crops – hydrolysis-sugar-fermentation 3) Lignocellulosics – separation-hydrolysis-sugar- fermentation; lignin as byproduct

• Thermochemical pathways to biofuels; 1) lignocellulosics – pyrolysis-bio oil-biofuels 2) lignocellulosics – gasification-methanol/FT-fuels

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Feedstock sources

• Forestry waste (forest residue, bark, wood chips, thinnings)

• Agricultural residues (straw, stover, bagasse)

• Energy crops (poplar, willow, switch grass)

• Municipal waste (paper, packaging,..)

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Biomass composition

Structure Softwood

(Picea abies)

Hardwood

(Betula verrucosa)

Wheat straw

Cellulose

Hemicellulose (C6-sugars)

Hemicellulose (C5-sugars)

Lignin

Extractives

Other components

42

19

7

27

2

3

42

4

26

23

3

2

38

1

24

24

3

10

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The ideal separation of biomass

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… and the reality

• Kraft and soda pulping

• Sulfite pulping

• Acid hydrolysis

• Steam explosion

• Organosolv pulping

At present, none of these processes results in an efficient and cheap separation

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Elemental analysis

Sample carbon hydrogen oxygen sulfur

Kraft lignin, pine

Kraft lignin, birch

Kraft lignin, E. globulus

Soda lignin, bagasse

Steam explosion, beech

64.3

63.5

56.1

61.8

57.6

6.0

6.1

5.7

6.0

6.0

27.9

28.0

35.4

32.2

36.4

1.8

2.4

2.8

0

0

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Substance Groups in Kraft Black Liquors(kg/ton of pulp)

Fraction Pine Birch

Lignin

Hydroxycarboxylic acids

Acetic acid

Misc. products

490

320

50

200

330

230

120

170

Ref: Sjöström 1993

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Principle for manufacturing of lignin from kraft black liquor

Black liquor

Evaporation

Precipitation pH = 9

Filtration,Washing

Lignin

Flash drying

Acid:CO2 or H2SO4

Filtrate,wash water

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Solvent fractionation of softwood kraft lignin

Fraction Yield Mn Mw Mw/Mn

CH2Cl2

n-propanol

Methanol

9

22

26

4.5 x 102

9.0 x 102

1.7 x 103

6.2 x 102

1.3 x 103

2.9 x 103

1.4

1.4

1.7

CH3OH/CH2Cl2

Undissolved

Unfractionated

28

14

100

3.8 x 103

5.8 x 103

1.4 x 103

8.2 x 104

1.8 x 105

3.9 x 104

22

31

28

Ref: Kringstad et al

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Lignin fractionation•Material: Industrial black liquor of softwood (pine/spruce), birch

and eucalypt respectively

•Fractionation: Ultra-filtration, 5 kD and 15 kD to remove high molecular particles / carbohydrates

•Lignin isolation: Precipitation with CO2 (pH 9), Acid washing with H2SO4 (pH 2.3), Drying•Purification: Cation-exchange to remove traces of Me+

Permeate

Retentate

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SEC of kraft lignins before/after fractionation

0 1 2 3 4 5 6

log M (relative polystyrene)

dw

/d l

og

M

SWL

SP5

SR5

0 1 2 3 4 5 6log M (relative polystyrene)

dw

/d lo

g M

EL

EP5

ER5

softwood eucalypt

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SEC-data from fractionated (5 kDa) kraft lignins

Sample/

polymer data

SW

lignin

SW

permeate

SW

retentate

Euc.

lignin

Euc.

permeate

Euc.

retentate

Mw

Mn

Polydispersity

5600

900

6.2

1800

450

3.9

6100

900

6.8

2300

530

4.4

1300

440

3.0

3400

660

5.1

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Thermal analysis of purified kraft lignins

Lignin sample/

thermal data

SW

lignin

SW

permeate

SW

retentate

Euc.

lignin

Euc.

permeate

Euc.

retentate

Tg, oC

Ts, oC

Td, oC

148

-

267

130

181

260

157

-

261

133

-

264

119

182

260

142

-

248

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Even a small lignin withdrawal can be interesting …

650,000 tonnes of pulp

Lignin withdrawal of 10% yields 33,000 tonnes

… converted to 16,000 tonnes of CF

…to support 160,000 cars with CF-composite (~40% replacement)

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Conclusions

• All native lignins are heterogeneous biopolymers linked to polysaccharides

• Alkaline or acidic processes result in both lignin degradation and re-polymerisation

• The up-grading of technical lignins require purification steps

• Several options exist for an increased lignin use